Non-alcoholic fatty liver disease (NAFLD) is the most common liver disorder with a prevalence estimated to be in 20-30% of the general population within the industrialized world. It is associated with a number of risk factors, including, type II diabetes, hepatocellular carcinoma (HCC), cardiovascular disease, and hyperlipidemia. MicroRNAs (miRNAs) are a new class of naturally occurring small non-coding RNAs that are known to play critical roles in a number of metabolic disorders, in part, by inhibiting expression of target genes. It is now well established that the introduction of specific miRNAs, or antimiRs into diseased cells and tissues can induce favorable therapeutic responses. Indeed, miR-122 inhibitor has entered phase III clinical trials as a therapeutic agent for HCV infection. Hepatocytes are the major cell type that controls lipid metabolism and the primary site of lipid accumulation in NAFLD. To address their potential role in the development of NAFLD, we initially identified miRNAs that are highly and specifically expressed in hepatocytes and found that one of them, miR-378/378*, is significantly induced in the livers of mice maintained on a high fat diet (HFD). Further, by inhibiting miR-378/378* we were able to significantly prevent hepatic lipid accumulation and hyperlipidemia in HFD-treated mice. Bioinformatic and validation studies revealed that miR-378/378* directly inhibited NRF1 (nuclear respiratory factor 1) and SORT1 (Sortilin 1), two gatekeepers of NASH and dyslipidemia, and induced expression of TNF?, a promoter of NASH. These findings led us to hypothesize that miR-378/378* promotes hepatic lipid accumulation and the progression of NAFLD to NASH by simultaneously modulating expression of NRF1, SORT1 and TNF?. The objective of this project is to determine the roles of the crosstalk of miR-378/378* with NRF, SORT1 and TNF? in regulating hepatic lipid accumulation and the progression of NAFLD to NASH. The long-term goal of this study is to elucidate the underlying mechanism(s) of NAFLD and its progression to NASH, and develop miRNA inhibitors as therapeutic agents for both disorders. Three Specific Aims are designed to test our hypothesis. Specifically, we will (1) establish gain-of-function studies for miR-378/378* in affecting hepatic lipids in HFD-treated mice, and determine whether their inhibition prevents hepatic lipid accumulation via modulation of NRF1 and SORT1; (2) determine the role of miR-378/378* in promoting progression of NAFLD to NASH, and elucidate the underlying mechanisms of this process; and (3) evaluate the levels of miR-378/378*, SORT1, NRF1 and TNF? in a set of liver samples from patients with a spectrum of NAFLD and NASH. Our studies are designed to establish miR-378/378* as a new pathway for the regulation of hepatic lipid metabolism and the progression of NAFLD to NASH. The combination of strong preliminary data and a logical and rationally based experimental design make this project highly feasible. The results will provide novel insights into the physiological roles and mechanisms of miRNAs, in addition to their potential therapeutic application for both of these hepatic disorders.